Many plant natural products have biological activities that make them valuable as pharmaceutical drugs. Alkaloids are a class of natural products that have proved particularly useful as drugs and medicines. Examples of biologically-active alkaloids include morphine, scopolamine, camptothecin, cocaine and nicotine. These compounds are all isolated from plant sources for use as pharmaceutical drugs. Nicotine, morphine (and related opiates) and cocaine are also addictive drugs that are responsible for significant health and societal problems worldwide.
Nicotine is a pyrrolidine alkaloid that exhibits a range of bioactivities, including potent toxicity and nervous system stimulation. In Nicotiana tabacum, N. benthamiana and a number of other species, nicotine is synthesized in the roots and then transported to the leaves, where it appears to play a role in defense. The biosynthesis of nicotine and many other plant metabolites can be induced by the application of a class of volatile plant hormones collectively termed jasmonates (Gundlach et al., Proc. Natl. Acad. Sci. U.S.A. 89: 2389-2393 (1992)). Although increases in nicotine levels can be induced by wounding or jasmonate application, the actual regulatory machinery responsible for this induction has yet to be discovered.
Plant natural product biosynthesis is mainly under transcriptional control, which allows plants to regulate metabolism in a developmental and stress-specific fashion. A number of transcription factors that regulate specific branches of secondary metabolism have been identified in plants. Anthocyanin biosynthesis is controlled by interacting MYB proteins (e.g. maize C1, Arabidopsis PAP1/PAP2) and basic-helix-loop-helix proteins (e.g. maize R, petunia AN1) (for a review see Vom Endt et al., Phytochemistry 61: 107-114 (2002)). Examples of other transcription factors regulating plant metabolic processes include a WRKY-type transcription factor that appears to control the transcription of a sesquiterpene synthase in cotton trichomes (Xu et al., Plant Physiol. 135: 507-515 (2004)) and an AP2/ERF-like transcription factor, WIN1, that up-regulates wax biosynthesis in Arabidopsis (Broun et al., Curr. Opin. Plant Biol. 7: 202-209 (2004)).
Overexpression of ORCA3 in Catharanthus roseus cell suspensions increased levels of transcripts of genes encoding some of the enzymes in the C. roseus terpenoid indole alkaloid pathway, but alkaloid accumulation was observed only when the cell suspension were provided with loganin, a terpenoid precursor. (van der Fits and Memelink. Science 289:295-297 (2000)). Overexpression of two transcription factors, NtORC1 and NtJAP1, increased transient expression of marker genes linked to a putrescine N-methyltransferase (PMT) promoter in tobacco cell suspensions. (De Sutter et al., Plant J. 44:1065-76 (2005)).